e1000: Restore VF interface random MAC

Restore 525e07418c77 after the iflib conversion of igb(4). This
reenables random MAC address generation when attaching to a VF with a
zeroed MAC.

PR: 253535
Reported by: Balaev PA <mail@void.so>
Reviewed by: markj
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D29785